The ultimate goal of the project is to understand in molecular detail the mechanisms regulating sexual development in Phycomyces. This system provides a fascinating example of mutual signaling between two types of cells. Such signaling may be the root of the organizing principle in mammals. The system also provides an example of elementary differentiation. Pheromones initiate sexual development by stimulating zygophore (sex cell)-development, carotenogenesis, their own biosynthesis, and possibly all other facets of sexual development. The pheromones made by one sex are active only in cultures of the opposite sex. The structures of 9 sex pheromones active in Phycomyces are known; their mechanisms of action are unknown. The mutational analysis outlined in this proposal provides a basis for obtaining mechanistic answers for some (if not all) aspects of pheromone function. Work centers upon screening for 120 mutants blocked in early sexual development and then identifying their defects behaviorally, biochemically, and genetically. This specific project: 1) will determine whether the sex pheromones or their metabolites, the trisporic acids, actually regulate zygophore-development, 2) will indicate the (minimum) number of events occurring between pheromone-receptor and zygophore-development, and 3) will identify the (early) sex-specific events. More importantly, this project will provide mutants which can be used subsequently in the analysis of: growth arrest, the regulation of pheromone biosynthesis, the regulation of zygophore-development, and (possibly) the molecular basis of sexuality. Furthermore, the overall project may reveal a novel regulatory mechanism, unrecognized to date, which could be present in all organisms. The more that is known about regulatory mechanisms at any level of biological organization, the better the prospects of discovering how abnormal mammalian cells function and of discovering how to bring them under control.